The present invention relates generally to so-called fluidized bed combustion and, more particularly, to certain new and useful improvements in fluidized bed boiler apparatuses and methods for carrying out and operating same.
Combustible materials, such as a variety of fossil fuels, have been combusted in fluidized bed apparatuses, such as boilers, to generate heat by passing a heat-exchange fluid, such as water, in a heat-exchange relationship through the fluidized bed to obtain steam from the water. Fluidized bed combustion is well-known and is and has been the subject of extensive development (see "Fluidized-Bed Combustion Review" by H. Nack et al., Batelle, Columbus, Ohio, presented at International Conference on Fluidization, June 1975, Asilomar, Calif., incorporated by reference herein).
Typically, a fluidized bed boiler has a combustion chamber in which a particulate fuel, such as coal, is introduced to a fluidized bed of particulate material by passing air through the bed to promote the combustion of the fuel. The fluidized bed generally includes inert particulate particles, and may also include dolomite, limestone or other materials which serve to absorb or to react with the sulfur or other undesirable additives in the fuel to be combusted. The fluidized bed is often quite shallow and has heat-exchange tubes immersed in the fluidized bed to effect heat transfer from the heated particles to the water passing through the heat exchange tubes. In some cases, additional tubes with water are placed in the space above the bed.
Fluidized bed combustion generally operates at temperatures of 1500.degree. to 1700.degree. F., which are milder conditions than those encountered in conventional nonfluidized bed boilers, and thus nitrous oxide gases are considerably reduced. Such fluidized bed boilers are quite useful, particularly with solid fuel, such as coal, since they eliminate the premixing of the air in the fuel and lead to lower temperature conditions, thus, eliminating slag deposits on the cooling surface, and at the same time providing for very high heat-transfer rate to the surface.
In the operation of various prior art fluidized bed combustion apparatuses, such as incinerators and boilers, a portion of the fluidized bed has been recirculated by various techniques to improve heat output or apparatus performance, such as, for example, the recirculation of a bed within an incinerator as set forth in U.S. Pat. No. 3,702,595, and the recirculation of a fluidized bed comprising a coal- and solid-absorbent material in a stacked fluidized bed arrangement, as set forth, for example, in U.S. Pat. No. 3,905,336, and the recirculation internally within a fluidized bed apparatus as, for example, in U.S. Pat. No. 3,910,235.
However, there are certain problems associated with known fluidized bed boilers and the pressure conditions under which they operate, particularly where heat exchange tubes are inserted into the fluidized bed. Such heat exchange tubes often obstruct the flow of the fluidized bed particles, present difficult support problems at the temperature conditions employed, and restrict fluidization of the bed by the creation of dead spots therein. Fluidization in such systems has been improved by increasing the air velocity through the bed of solids, but this creates a further problem in that the finer bed particles tend to be carried out of the fluidized bed. To prevent excess fluidized bed particle carryover, larger-size particles in the fluidized bed have been employed; that is, particles with an average particle size of about 500 microns, and typically 300 to 450 microns in size, in comparison to the usual average particle range of about 100 to 150 microns. The employment of larger particles, however, reduces the heat-transfer rate due to the reduced surface area per given weight of the larger-size particles, and also creates unsteady flow conditions in the bed, resulting in large bubble formations, thereby reducing the efficiency of contact and efficiency of combustion. Therefore, in most fluidized bed boilers, a compromise is employed wherein velocity, particle size, operating pressure and the use of the form, number and shape of the heat exchange tubes placed in the bed are balanced to arrive at a compromise on the heat efficiency desired in the particular boiler.
Another drawback of prior art fluidized bed boilers is the relatively low number of control points for enabling various parameters, such as temperature and particularly, operating capacity, to be controlled. In addition, current proposals for increasing overall capacity include stacking several fluidized beds over each other, while slumping some of the beds to control capacity. However, not only is such configuration severely limited by vertical dimension constraints but there will also be structural support restrictions, particularly in view of the operating temperatures, all of which serve to limit the practicality of this approach.
Another proposal involves passing fluidized bed particles to a chamber having heat exchange tubes positioned therein. However, the tubes tend to obstruct flow and, more importantly, a gas bubble-like layer builds up on the surface of the tubes rather than a continuous mixture of solids and gas, thereby substantially lowering heat transfer efficiency since much greater heat transfer is provided by solids contact than gas at the same temperature.
Furthermore, prior art devices are generally inacapable of utilizing the "Fines", or finely ground solid fuel (e.g., coal) particles, produced during the crushing operation. Thus, efficiency is further reduced to the extent the "Fines" are lost with the flue gas or are not used at all.
Accordingly, it is an object of the present invention to provide a new and improved fluidized bed boiler apparatus and method for carrying out and operating of same. Another object of the invention is to provide a new and improved fluidized bed boiler apparatus and method for carrying out and operating same, capable of relatively high efficiency heat transfer, yet operating at relatively low temperatures to limit production of nitrous oxide and enabling suitable reaction for removing SO.sub.2.
It is also an object of the present invention to provide a new and improved fluidized bed boiler apparatus and method for carrying out and operating same, capable of relatively high heat transfer with no slagging of the ash.
It is an additional object of the invention to provide a new and improved fluidized bed boiler apparatus and method for carrying out and operating same, which enables finely ground solid fuel particles otherwise too small to burn in a fluidized combustion bed, to be completely combusted and the heat generated thereby to be efficiently utilized.
It is still another object of the instant invention to provide a new and improved fluidized bed boiler apparatus and method for carrying out and generating same, which includes a relatively large number of control points for enabling certain operating parameters to be controlled, particularly the operating capacity of steam generation.
It is also a further object of the invention to provide a new and improved fluidized bed boiler apparatus and method for carrying out and generating same, wherein particulate matter circulates in heat exchange means for increased heat transfer.
It is still an additional object of the invention to provide a new and improved fluidized bed boiler apparatus and method for carrying out and operating same, which enables control of the temperature of the fluidized bed.
Objects and advantages of the invention are set forth in part above and in part below. In addition, these and other objects and advantages of the invention will become apparent herefrom, or may be appreciated by practice with the invention, the same being realized and attained by means of the instrumentalities, combinations and methods pointed out in the appended claims. Accordingly, the present invention resides in the novel parts, constructions, arrangements, improvements, method and steps herein shown and described.